Adjustable time delay explosive charge element



March 8, 1966 H, M, LANG 3,238,871

ADJUSTABLE TIME DELAY EXPLosIvE CHARGE ELEMENT Filed Nov. 12, 1963 HAROLD M. LANG l 3 INVENTOR.

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United States Patent O 3,238,871 ADJUSTABLE TIME DELAY EXPLOSVE CHARGE ELEMENT Harold M. Lang, Tulsa, Okla., assignor to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed Nov. 12, 1963, Ser. No. 322,759 7 Claims. (Cl. 102--20) This invention relates to explosive charges suitable for use in seismic surveying, and particularly, it relates to an improved time-delay explosive element for use in generating seismic waves.

l-Ieretofore, elongated distributed explosive charges have been proposed for use in seismic surveying, wherein the eflective detonation velocity of the charge is matched approximately to the seismic wave transmission velocities of the medium surrounding the charge. Such distributed charges have been found very useful in discriminating against the generation of seismic noise and in preventing undue damage to the surrounding medium. An example of the above-mentioned distributed charge is disclosed in Silverman U.S. Patent 2,609,885, wherein an explosive charge is constructed employing a helix of a linear explosive, such as Primacord detonating fuse, the turns of the helix having a pitch providing an eifective detonation velocity approximately equal to the seismic wave transmission velocity of the surrounding formations in the direction of the detonation. Typically, such explosive charges are fabricated of an elongated mandrel having a helical groove formed in it from an end to end, with the turns of linear explosive fitted into the groove. This type charge has been found satisfactorily in the higher velocity ranges, e.g., above about 6,000 feet per second. However, in lower velocity ranges the reliability has been found to be less than is desirable. In the latter instance, failure frequently occurs because of damage to the turns of linear explosive immediately ahead of the detonation front. Such damage may prevent the propagation of detonation along the entire length of a mandrel or along a series `of mandrels joined together end to end. Because of the need for certain minimum charge weights and practical limitations of the winding of the linear explosive on the mandrel, it is not possible to obtain satisfactory reliability of this type charge in many instances.

It is also known, as disclosed in Silverman U.S. Patent 2,770,312, to utilize a combination of different explosive charges having varying detonation velocities to provide an over-all average detonation velocity approximately matched to the transmission velocity of a formation. However, with such explosive charges it is diflicult to adjust the average detonation velocity in the field to provide a suitable explosive useful under varying conditions, and to attain a satisfactory low velocity charge.

The present invention provides an adjustable time-delay explosive charge wherein the time required for the detonation to proceed from end to end may be readily varied to provide a preselectedV effective velocity of detonation in a controlled velocity explosive charge. Briefly, the present invention employs a linear explosive having a fixed length and known velocity of detonation, with means for varying the length of the detonation path along the linear explosive and t-hus vary the time required for detonation to proceed from one end of the linear explosive to the other end.

In accordance with the invention, there is provided an adjustable time-delay element for use in the generation of seismic wave signals which element comprises an elongate mandrel, a first linear explosive section extending along the mandrel, a second linear explosive section extending along the mandrel and means for bridging be- Patented Mar. 8, 1966 tween the sections at multiple points to provide a variable length detonation path along the element. In a preferred embodiment of the invention of the linear explosive is in the form of a generally U-shaped conguration with the legs thereof extending along the mandrel and a carriage member, with an explosive bridge component, slidably interconnecting the legs while maintaining the explosive bridge component in detonating contact with both the legs. Such a time-delay element is adapted to be coupled into a string of explosive charges so that it receives a detonation from a preceding unit, consumes a predetermined time to propagate its own detonation from end to end an-d then transmits the detonation to a subsequent unit to provide the desired over-all detonation velocity.

The present invention will be better understood by reference to the following description of a preferred enibodiment of the invention and to the accompanying drawings wherein:

FIGURE 1 schematically illustrates a cross-sectional view of a preferred adjustable time-delay explosive charge unit; and

FIGURE 2 schematically illustrates the time-delay unit of FIGURE 1 coupled into a string of explosive charge units employed in a shot hole for the generation of seismic wave signals.

Referring to FIGURE l, a preferred time-delay unit 11 utilizes an elongate cylindrical mandrel l2, which typically may be a hollow tube made of plastic, ber, cardboard or like materials haveing suitable properties of strength and rigidity to enable it to be coupled into a string of explosive charges lowered into a shot hole. The length of the mandrel, of course, may very considerably, and typically are 3 to 10 feet in length. However, units approximately 5 foot in length are normally used. The mandrel diameter also may be varied widely, with typical units employing a mandrel having a diameter of approximately 11/2 inches. Mandrel 12 may be provided at either end with screw-type couplings 13 and 13a to enable the time-delay unit to be interspersed into a string of explosive charges utilized for generating seismic signals.

Supported by mandrel 12 and extending along the length thereof is a linear explosive 14 having a preselected fixed length. The linear explosive 14 advantageously comprises a generally U-shaped conguration, with the legs of the U shape extending along the wall of the mandrel, substantially parallel to one another. The upper end of the linear explosive 14 may be provided with a small input booster 16 to aid in transferring detonation from the preceding explosive unit to the time-delay unit, and similarly, the lower end of the linear explosive may be provided with a small output booster 17. The linear explosive is brought into mandrel 12 through a central opening in coupling 13a, and a protective tube 18, made of polyethylene vinyl plastic, or the like may be employed to protect and stiften the linear explosive. Similarly, the lower end is brought out of the mandrel 12 through coupling 13 and protective tube 19 is employed on the lower end as described above. Outer sheath 15 is employed to protect and secure linear explosive sections 24 and 25 on the outside of the mandrel.

The total length of linear explosive 14 will vary according to the detonation velocity of the linear explosive and to the required degree of flexibility in time adjustment. Those portions ahead of and following the U- shape, sections 24 and 25, respectively, may tbe varied in length to provide a desired propagation time, while within that portion constituting the U-shape, the detonation path and, consequently, the time may be varied. In the embodiment shown, the length of the detonation path along the linear explosive may be varied by changing the position of bridging charge 21 along legs 20 and 20a of the U-shape. Advantageously, bridging charge 21 is mounted on movable carriage 22 which slidably interconnects the sections, or legs of the U-shape. Carriage 22 is provided with guide members 22a and 22b, through which is threaded the linear explosive. Carriage 22 slides along substantially the complete length of each leg, while maintaining the bridging charge in detonating contact with each of the legs. Thus, as detonation is initiated in the linear explosive and proceeds along leg 20a of linear explosive 14 it will arrive at bridge 21 and the detonation transferred across the bridge to the other section 20 of linear explosive and proceed along the length thereof to the lower end of the linear explosive. By changing the position of bridge 21, the detonation may be short-circuited or shunted across the sections of the U-shape at any position along its length so that the detonation path is varied, and consequently, the time required for detonation to proceed along the length of the linear explosive will be varied. Of course, upon reaching bridge 21, the detonation may also proceed past the bridge location, but where a very low-power linear explosive is employed no significant interfering seismic noise results. Detonation of the short-circuited portion of the explosive will occur, and then cease when it reaches the opposite side of the bridge.

Advantageously, the movable carriage 22 may be connected to a cord-like element 23 extending from eye 40 to the exterior of mandrel 12 so that by withdrawing cord 23 the carriage is moved. If desired, calibration marks may be printed along the outside of mandrel 12 to aid the operator in moving the carriage to obtain the required time delay.

Linear explosive 14 preferably is formed `of a lowpower explosive, such as Primaline detonating fuse having an explosive load of about 4 grains per foot and a velocity of detonation of about 17,000 -feet per second. However, other similar materials `formed of cords or flexible tubes containing particles of pentaerythritol tetranitrate (PETN), cyclotrimethylenetrinitramine (CRDX) or like explosives of varying Weights and velocities are commercially available and many of these may be employed in the practice of the invention. Advantageously, the linear explosive employed in the timedelay unit has essentially no seismic power, i.e., it produces no significant seismic wave signals detectable by the seismometers used under the circumstances wherein employed, and thus causes little or no interfering seismic noise. Further, the detonation shock eld surrounding this small linear explosive is such that it may be placed relatively close to itself without danger of the cord cutting itself or damaging other elements by its own detonation wave. Consequently, a linear explosive having an explosive load of less than about l grains per foot is preferred.

The explosive material utilized in bridging charge 21 may be any suitable explosive capable of rbeing detonated by linear explosive 14 and of detonating the linear explosive used. Preferably, it is formed of the same material as the linear explosive.

The following example will illustrate the manner in which the above-described time-delay unit may be used in conjunction with explosive charges in generating seismic wave signals. If a vertical helix of 60 grains per foot Primacord (or other suitable linear explosive) is wound on a ll/z-inch diameter mandrel with a pitch of 1% inches, the resulting charge will have an effective detonation velocity of approximately 8,000 feet per second. If such distributed charges are to be employed as the power charge in a shot hole traversing a formation having a seismic wave transmission velocity of 5,000 feet per second, it is necessary to delay the detonation of the power charge by a calculable amount. Thus, if -foot poles of the power charge units are employed and lO-foot intervals of the formation of interest are considered, one- CII 5-foot power pole may be employed in conjunction with one 5-foot delay unit pole to provide the required propagation time. The time required for the detonation to proceed along the 10-foot interval, at a velocity of 5,000 feet per second, is 2 milliseconds. The 8,000 feet per second power pole requires 0.625 millisecond for detonation to proceed its length, and this leaves a remainder of 1.375 milliseconds which must be occupied as the detonation proceeds along the delay unit. With a Primaline linear explosive utilized in the delay unit, at a detonation velocity of 17,000 feet per second, approximately 22 feet of Primaline must then be traversed in the detonation path of this pole. If the above-described U-shaped configuration is employed on a S-foot mandrel, approximately 9 feet of the total length of linear explosive may be occupied by the adjustable loop, with approximately 13 feet provided above and/or below the U-shape. In such an instance the cord 23 is withdrawn to its full length, placing the bridge member 21 at the top end, or the closed end, of the U-shape. With the bridge in this position, the detonation must traverse the entire length of the linear explosive, which requires approximately 1.375 milliseconds.

If the next succeeding `formation of interest is found to have a seismic wave transmission velocity of 7,500 feet per second, the above units will require a delay of 1.33 milliseconds `for each 10-foot interval. As mentioned, 0.625 millisecond is required for the 5-foot long, 8,000 feet per second power charge, and the time-delay unit will need to furnish a delay of only 0.7 millisecond. Consequently, the length of the detonation path in the delay unit should Ibe approximately 13.5 feet, which may be attained by positioning bridge member 21 at the lowest position on the U-shape so that detonation of the Primaline proceeds from its upper end, along section 24 to its lower end, and then across bridge 21 to section 25, from whence it proceeds to the lower end of the delay unit.

As shown in FIGURE 2, one or more delay units 11 may be interspersed in a string of explosive charges 31 for insertion into a shot hole 30 to generate seismic wave signals. Typically, unitized lengths of the explosive charges are coupled together as shown, so that detonation from the upper unit is transferred to the delay unit to follow the selected detonation path therein, and then transferred to the succeeding lower unit. Power units 31 may comprise the aforesaid helical distributed charges and/ or other suitable explosives, such as cans of Nitramon primer or the like, so that the desired total charge weight is provided in the string. As shown, an electrical blasting cap 32 attached to upper unit 31 and connected by electrical leads 33 to a suitable power source, such as blaster 34, may be used to initiate the detonation in the explosive string so that the detonation proceeds down the string in the manner referred to above. As described above, the desired length of the detonation path in delay unit 11 may be Selected by withdrawing cord 23 to position the movable bridge as desired. When unit 31 is threaded into the top of delay unit 11, cord Z3 is firmly held by the coupling joining the two units so that the movable bridge is secured in place.

It will be understood that the foregoing description has been given only for exemplication and is not intended to limit the scope of the invention. From the foregoing description, various modications and alterations in the invention will become apparent to the skilled artisan, and as such, these fall within the spirit and scope of the invention. For example, various configurations, other than a U-shape may be utilized, and the bridging charge may shunt the detonation across a larger number of sections or the bridge may comprise one or more removable pins serving to bridge between sections of linear explosive.

I claim:

1. An adjustable time-delay element for use in the generation of seismic wave signals which element comprises` a mandrel, a length of linear explosive extending along said mandrel, said linear explosive being in the form of a U-shaped configuration with the legs thereof extending along said mandrel substantially parallel to one another, a carriage member interconnecting said legs and slidable along the length thereof, and an explosive bridge component carried on said carriage in detonating Contact with both of said legs, and means for moving said carriage member along said mandrel to vary the detonation path along said linear explosive, whereby the total time required for detonation may be varied.

2. An apparatus as defined in claim 1 wherein said linear explosive has substantially no seismic power.

3. An explosive charge for use in generating seismic wave signals comprising a string of explosive power units each having known effective detonation velocities, and at least one time delay explosive charge element interspersed in said string, said time delay element including a mandrel; a length of linear explosive extending along said mandrel, said linear explosive being in the form of a U- shaped configuration with the legs thereof extending along said mandrel substantially parallel to one another, a carriage member interconnecting said legs and slidable along the length thereof; an explosive bridge component carried on said carriage in detonating contact with both of said legs; and means for moving said carriage member along said mandrel to vary the detonation path along said linear explosive whereby the total time required for detonation may be varied.

4. An apparatus as defined in claim 3 in which said linear explosive has substantially no seismic power.

5. A time delay explosive lcharge element for use in the generation of seismic wave signals, which element comprises an elongate linear explosive having a known velocity of detonation, the ends of said linear explosive being a fixed distance apart and at least a part of said elongate linear explosive being in the form of a U-shaped configuration with the legs thereof substantially parallel to one another, said legs being in a fixed relation to each other and spaced a sufcient distance apart to prevent cross detonation; an explosive bridge means interconnecting said legs and slidable along the length thereof; and means for moving said explosive bridge means along said parallel legs from one end thereof to the other, whereby the total time required for detonation from one end of said elongate linear explosive to the other may be varied.

6. A time delay explosive charge element for use in the generation of seismic wave signals, which element comprises an elongate mandrel, an elongate linear explosive having a selected total length and a known velocity of detonation, said linear explosive having relatively straight sections parallel to each other and to said mandrel, two of said sections being of a continuous linear explosive fixed with respect to each other; an explosive bridge means interconnecting two of said parallel sections in detonatable relation and slidable along the length thereof; and means for moving said explosive bridge means along a length of said parallel sections to vary the length of the detonation path along said linear explosive whereby the total time required for detonation to travel from one end of said explosive to the other may be varied.

7. A time delay explosive charge element as defined in claim 6 in which the ends of said elongate linear explosive are spaced longitudinally from each other on opposite sides of said explosive bridge means.

References Cited by the Examiner UNITED STATES PATENTS 993,091 5/1911 Meigs et al. ,102-852 1,319,636 10/1919 Wiley IGZ-85.2 X 3,131,633 5/1964 Eisler et al. 102--2l.6

FOREIGN PATENTS 525,171 5/1955 Italy.

BENJAMIN A. BORCHELT, Primary Examiner.

SAMUEL FEINBERG, Examiner. 

1. AN ADJUSTABLE TIME-DELAY ELEMENT FOR USE IN THE GENERATION OF SEISMIC WAVE SIGNALS WHICH ELEMENT COMPRISES A MANDREL, A LENGTH OF LINEAR EXPLOSIVE EXTENDING ALONG SAID MANDREL, SAID LINEAR EXPLOSIVE BEING IN THE FORM OF A U-SHAPED CONFIGURATION WITH THE LEGS THEREOF EXTENDING ALONG SAID MANDREL SUBSTANTIALLY PARALLEL TO ONE ANOTHER, A CARRIAGE MEMBER INTERCONNECTING SAID LEGS AND SLIDABLE ALONG THE LENGTH THEREOF, AND AN EXPLOSIVE BRIDGE COMPONENT CARRIED ON SAID CARRIAGE IN DETONATING CONTACT WITH BOTH OF SAID LEGS, AND MEANS FOR MOVING SAID CARRIAGE MEMBER ALONG SAID MANDREL TO VARY THE DETONATION PATH ALONG SAID LINEAR EXPLOSIVE, WHEREBY THE TOTAL TIME REQUIRED FOR DETONATION MAY BE VARIED. 